A practical way to double the bit carrying capacity of an existing unidirectional fiber optic communication link is by the use of optical circulators. An optical circulator is a passive, non-reciprocal device, which permits full duplex communication on a single fiber optic link. Thus, a typical fiber optic communication link operating on two fibers can be quickly and economically converted to a bi-directional, single fiber communication link by installing an optical circulator at each end of the link.
One of the major advantages of optical circulators over more traditional 3 dB couplers is that the loss penalty is much lower. Using a 3 dB coupler at each end of a fiber link, there is an insertion loss of at least 6 dB. For connections that operate near their detection limits, this additional 6-dB loss could make bidirectional communication impracticable.
In a real optical circulator insertion loss and cross-talk as well as simplicity and cost are important considerations. Insertion loss is the difference in power between light launched into the optical circulator and the power that exits the device. Insertion loss is largely due to absorption of light and to coupling loss.
Prior art optical circulators are described in U.S. Pat. No. 4,650,289, issued to Kuwahara; U.S. Pat. No. 4,464,022, issued to Emkey; and in U.S. Pat. No. 4,859,014, issued to Schmitt et al. However, optical circulators made as described in these references relatively costly due to the large size of optical components required. For example, optical circulators generally require collimated beams, or lensing that will achieve collimation. However, a typical collimated beam in prior art circulators has a beam diameter of .about.350 .mu.m, and when a beam is split into two beams having two polarization states, components such as birefringent crystals are required that are sized to accept beams of such diameters.
One such optical circulator where separate lenses are required is described by the applicant, Cheng, in U.S. Pat. No. 5,471,340. Here three sequential ports are described as being arranged in series such that light input into a first GRIN lens is directed to a second GRIN lens, and light input into the second GRIN lens is directed to a third GRIN lens. Aside from the obvious drawback of this device being quite large in size due to the diameter of the beams provided by the GRIN lenses, the birefringent crystals must be large, and sized to accept these large beams. In fact it is estimated that the cost of materials of an optical circulator such as Cheng's wherein light is collimated at the input of the device, is approximately 50 times the cost of the materials required for the device in accordance with this invention. Hence there is a significant advantage in both the small size and lower cost associated with the device of this invention.
Furthermore, there are other advantages that may be viewed as quite unexpected. The device in the applicant's earlier U.S. Pat. No. 5,471,340 requires for each lens to be individually adjusted to achieve optimum coupling. Furthermore, each of the three lenses must be affixed to the rutile crystal it is coupled with. In the instant invention, a single fibre tube housing three preset optical fibres is affixed to a birefringent crystal, and tuning or adjustment for optimum coupling can be achieved conveniently by, aligning port 1 to 2 and then ensuring that the beam shift direction of the second rutile crystal is parallel to the line extending through the fibre array. In this manner all the ports automatically align.
It is also an object of the invention to provide a device that has lower insertion loss and cross-talk as well as simplicity in the optical circulator. Although some prior art devices use a folded configuration, this invention provides a folded configuration that only requires a single lens and small birefringent crystals and other optical components. Aside from reducing the size of the device considerably which has a significant commercial advantage, the cost is correspondingly reduced.
A more recent prior art optical circulator that uses birefringent crystals in contact with other elements such as polarization rotators, is U.S. Pat. No. 5,204,771 issued Apr. 20, 1993 in the name of Koga. Although this invention appears to perform its intended function adequately, Koga's configuration results in a large device compared with the folded configuration of the present invention. Furthermore, Koga requires large components in contrast to this invention.
It is therefore an object of the invention, to overcome many of the limitations of known prior art devices.
It is a further object of the invention to provide a compact optical circulator that is of a folded configuration.